Curable silicone compositions are used in the sealant, protective coating material, etc., of semiconductor elements in optical semiconductor devices such as light emitting diodes (LED). Examples of such curable silicone compositions include a curable silicone composition comprising: a diorganopolysiloxane having an average of two or more silicon-bonded alkenyl groups in a molecule; a resinous organopolysiloxane with a number average molecular weight of 2,000 to 5,000, having alkenyl groups, and comprising siloxane units represented by the formula: SiO4/2 and siloxane units represented by the formula: R2R′SiO1/2 (in the formula, R represents a monovalent hydrocarbon group lacking aliphatic unsaturated bonds, and R′ represents an alkenyl group); an organohydrogenpolysiloxane having an average of three or more silicon-bonded hydrogen atoms in a molecule; an adhesion promoter; and a hydrosilylation reaction catalyst (see Patent Document 1), and a curable silicone composition comprising: a diorganopolysiloxane having at least two silicon-bonded alkenyl groups in a molecule; a resinous organopolysiloxane comprising: siloxane units represented by the formula: SiO4/2, siloxane units represented by the formula: (CH2═CH)R2SiO1/2 (in the formula, R has the same significance as above), and siloxane units represented by the formula: R3SiO1/2 (in the formula, R has the same significance as above); an organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms in a molecule; and a platinum metal catalyst (see Patent Documents 2 and 3).
While a cured product with little surface tack, moderate hardness and moderate strength can be derived by increasing the content of resinous organopolysiloxane in such a curable silicone composition, the resulting composition still suffers the defect of markedly high viscosity as well as low fluidity and packing properties.
In order to inhibit the viscosity rise and to enhance the fluidity and packing properties, Patent Document 4 proposes a curable silicone composition comprising: a diorganopolysiloxane having at least two alkenyl groups in a molecule; at least two types of resinous organopolysiloxanes having different mass average molecular weights and comprising: siloxane units represented by the formula: SiO4/2, siloxane units represented by the formula: R2R′SiO1/2 (in the formula, R and R′ have the same significance as above), and siloxane units represented by the formula: R3SiO1/2 (in the formula, R has the same significance as above); an organopolysiloxane having at least two silicon-bonded hydrogen atoms in a molecule; and a hydrosilylation reaction catalyst.
However, because of the high gas permeability of the cured product, these curable silicone compositions suffer sealant discoloration and a decline in luminance due to the corrosion of silver plated on the LED board brought about by corrosive gas when these curable silicone compositions are used as sealants of high brightness LED with great heat generation and great optical strength. An additional problem is the decline in the optical output efficiency when large quantities of fluorescents are blended in these curable silicone compositions for use as LED sealants.